Anti Dive front geometry

kmead

kmead

Old enough to know better
So this is a bit of an arcane question, likely one answered by few and most likely by Steve Hoelscher.

I have noticed a fair amount of dive under braking with my X. It suggests the general front geometry has little if any anti dive built into the front suspension geometry.

Anti dive is a means of configuring the suspension axes of rotation such that a line through the lower arm front and rear mounts intersects with line a line perpendicular to the strut inclination and have that intersection be near and forward the CG of the car versus near the ground and behind the CG. This has little effect on the car in driving straight or turning but once braking torque is applied when you hit the brakes it helps resist the front diving down by turning the brake torque into a rising force.

On a Macpherson strut type supension this is generally accomplished by raising the rear lower a arm mount and or lowering the front mount. On an X raising the rear mount is quite difficult (early Porsche 911s accomplished it by changing the unibody to raise the rear mount into the body such as on the 930) however the front mount for the radius rod could actually be lowered to rotate the angle counter clockwise and induce some anti dive.

So my question, way down here, is has anyone done this on an X, lowered the front radius arm mount such that it is lower? If so by how much and what was the effect?

Thanks.
 
Karl,

You are provoking me... ;):D

This has actually be done and there is quite a bit of data available. If you have read/reviewed Project X1/9 and noted the relocation of the control arm mounts you will see that the original builder of the subject car raised the control arm mounts on both ends of the car. The radius arm body mounts were also revised by the location was not raised as significantly as the control arm mount was. Therefore there was additional anti-dive added to both ends of the car. I have driven a couple of cars that were built from the Project X1/9 manual and had the suspension mounting revisions.

The relocation of the control arm mounts has long been my biggest critique of Project X1/9. There are two flaws in that design; First is the roll centers were raised excessively. That was done at least in part due to the nature of the tires available at the time. Modern radial racing tires need a lower roll center to work effectively and the lower roll center improves mechanical grip. Second, the additional anti-dive geometry that was added significantly degraded braking performance. Its important to note that there was no real acknowledgement of the additional anti-dive in the text. So either they didn't consider it or dismissed it entirely.

For the average street car, 100% anti-dive is an undesirable trait. It provides no feedback to the driver and makes the brakes very unforgiving on anything less than a perfectly flat road surface. The more anti-dive the suspension has the less compliant it is to road surface irregularities under braking. It also tends to reduce total braking effect. The effect is very much like having very stiff suspension, with a very high roll center, when cornering. The car produces less mechanical grip due to the high roll center and excessive weight transfer due to lack of compliance. Same for lots of anti-dive; there is lots of weight transfer to the front suspension but little compliance. So its very easy for the driver to lock a front wheel due to lack of feedback and suspension compliance.

The X is designed with a little anti-dive so the car is very compliant under braking. This means its less likely to lock a front wheel under braking _and_ less likely to lock a _rear_ wheel under braking, all while providing excellent brake effect.

Much of the dive you are experiencing under braking comes less from the lack of anti-dive and more from the very soft front springs. The same soft springs that allow for all of that body roll. That body roll isn't the result of a low roll center but instead the very soft springs.
 
OK so it does have some anti dive in the front suspension.

I am just curious if a bit more would allow the rest of the suspension to continue to be compliant and reduce the level of dive.

On the 911 forums they have solutions that just lower the front of the a-arm, they do have other approaches which make significant changes to the front end more like the Project X book does.

The lowering of the front of the A arm is the reason for my question about lengthening the radius rod’s location downward and not changing anything else in the front suspension. I was thinking nothing radical, an inch or so which would have an effect but not be a huge change.

So wrong approach in your mind? It would be better to go to stiffer front springs and gain the change that way?

I do have to say I really appreciate the compliance of an X1/9 suspension, I am also not particularly bothered by lean as all the cars I grew up with have always had lean. My 82 Scirocco had no antiroll bars and would not only ‘tricycle’ around an autocross course but would also nearly lift the inside front wheel off the ground.

Yesterday I spent many hours driving the X on a drive with my local club on a wide variety of roads. At the end I still felt good and had a great time. In the past I have driven the same route in my wife’s Miata and at the end have been left feeling beaten and tired.

The X despite leaning a great deal still manages to be a fun car at an autocross and even better a great car to drive for hour after hour on varied roads, its really its best feature and I don’t want to spoil it.

Thanks
 
I have quite a bit of experience with Porsches, mostly the 996, 997, 991 GT3 Cup and GT3-RSR cars, the 996-997 chassis cars continued the strut front suspensions. Of course these are much more sophisticated than the 993 and earlier chassis, despite the strut designs.

Porsche designed in more anti-dive due to modern aero requiring a more stable platform to maintain its effect. Again, its a balancing act. Maintaining the aero's performance envelope while providing the driver with adequate feedback and braking compliance. Too much anti-dive and you will improve aero performance but compromise the compliance and thus lengthen the braking distances as the driver tries to manage the braking. Modern, racing, ABS systems are of considerable benefit here as it can manage the threshold but low compliance will still lengthen stopping distances.

All of that aside, back to your X.

The 60s~80s era 911s are not unlike the X1/9. Suspensions of that period were soft due to the nature of tires, roads and design standards. The rear engine (and weight bias), road going, 911s had very soft front springs to maintain ride quality and anti-dive geometry similar to the X1/9, which promoted front brake compliance. So increasing anti-dive percentages are desired for the same reason you noted in the X.

Certainly you could add some anti-dive and flatten out the car's braking dynamics and as long as you don't get extreme you might find reasonable results. I think it would be pretty easy to make a spacer to fit between the radius arm mount and the body. This would make testing simple and easy. As I noted above I would suggest trying a stiffer front spring. I always fitted my street cars with stiffer front springs, a pair of rear springs (larger wire diameter), with a coil and a half removed, worked nicely. The additional spring rate reduced both body roll and improved front end response with only minimal degradation in ride quality. a spring swap is also easy to test, although you will probably need to reset the front toe.

As with all mods affecting ride quality, results are subjective; what I might like you might find intolerable. My advise and recommendations are based on my preferences. Your mileage may vary.
 
Thanks Steve, I really appreciate the depth of knowledge you impart from your real experience with these and other cars.

I will cogitate a bit on this and think through some solutions and play out where I think I might be with my car. Perhaps some drawings over the cutaways will assist me in understanding how much to change height of the radius rod and what it’s effect would be before actually changing the car.

Thank you.
 
I use a suspension software modeling tool when I am working on whatever racecar I am currently working with. There are existing models for the Porsches and the various GT3 and GT4 racecars I deal with regularly. There wasn't a model for the MR2 when I started with it so I went to the time and effort to take measurements of the chassis and make my own, not something I would want to do again. I used a much simpler tool when I was working with my X1/9 which was quite crude, really only having roll center/axis heights and the resulting roll couple. It was useful and I learned a lot from it but I never really experimented with anti-dive changes on my own car.

A quick note about brakes relative to the X1/9. A stock X1/9 has great brakes, assuming they are working normally. This in part due to the amount of front suspension compliance. But there are a number of contributors. Most people will geek out about things like "swept area", pad surface, caliper area and heat dissipation. And I from time to time see people wanting to upgrade their brakes. However, brakes are application specific. Unless you have something like a turbo conversion or a K20 swap the OE brakes are probably more than adequate with a simple pad upgrade. I will use my old DSP autocross car as an example.

Reviewing the MPH data from a typical autocross run from the X1/9 shows the typical speed delta to be about 30 mph, from a minimum of 30 to a maximum of 60 mph. The in-line g data (braking and acceleration) shows the typical braking event to be under 1.5 seconds. The brief nature of the typical braking event is due to the car's lack of straight line acceleration and its relatively light weight and the car's ability to sustain more than 1g of braking. Given that brakes are really just a heat sink (they convert inertia to heat through the friction of the pad on the rotor) the OE brakes, with an appropriate pad compound, are more than adequate for the application.

Now, given that the OE braking system is capable of 1g+ of braking effect and absorbing and dissipating the heat energy produced, the only thing that is left is the tire's grip and chassis compliance to the braking force. Increasing the tire's grip will reduce the braking event for a given delta. Or, changing the chassis' compliance. More anti-dive reduces chassis compliance increasing weight transfer and reducing overall brake effectiveness.

Yes, you can have too much as well as to little anti-dive. To little results in front brake lockup and too little can also produce too much weight transfer but in this case causing rear brake lockup. So there is definitely a sweet spot for anti-dive on every chassis and every application.
 
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Like all things, the balance achieved is the sum of all the component parts. The front dive does rotate weight onto the front wheels and as you state unloads the rear wheels. The balancing of the whole system of the car is what we love.

I agree that the X brakes in working condition work remarkably well. I often have to bring it down quickly to stop for a light as some of our yellow’s are rather short or some less than stellar driver and it does indeed work well. My brakes have been reliable on my X’s (the 850 a bit less so) with the exception of the quality and capability of the rear acting emergency brake solution. The reliability particularly now that I would never drive these cars in snow and salt unlike my second X.

All that said, there has been an ongoing desire amongst us to change the rear brakes to a larger diameter piston, akin to the ones the Scorpion uses, 38mm versus 34mm of OE. A seemingly subtle change.

One of my many projects for later this year is to look into adapting floating pin type calipers from more modern cars to improve future part availability and . The rear brakes I am looking at come from a VW Golf/Jetta and have a 38mm caliper and a rear disc which is nearly the same diameter (a few mm different) which should allow the Fiat discs to work fine. Information, not the question.

So here’s the question(s), have you considered or actually changed your rear calipers on your X to increase the rear bias? With the rear weight bias an X possesses, it would seem that increasing the percentage of braking by the rear brakes would be a worthwhile change. I clearly understand the issues around the possibility of rear lockup having lived through the GM X car times and more recently experiencing a test drive of a VW Rabbit whose ABS system had failed.

There was a thread on parts of this a while ago where I learned a great deal from Bernice and a few others (which I sorely needed) relative to force multiplication.

Given that larger rear pistons would move the brake bias rearward and increase the tendency towards rear locking, with a bit more anti dive would it counter the front wieght transfer and assist in avoiding rear lockup with a more rear biased brake system?

Thanks
 
kmead said:
So here’s the question(s), have you considered or actually changed your rear calipers on your X to increase the rear bias? With the rear weight bias an X possesses, it would seem that increasing the percentage of braking by the rear brakes would be a worthwhile change. I clearly understand the issues around the possibility of rear lockup having lived through the GM X car times and more recently experiencing a test drive of a VW Rabbit whose ABS system had failed.

There was a thread on parts of this a while ago where I learned a great deal from Bernice and a few others (which I sorely needed) relative to force multiplication.

Given that larger rear pistons would move the brake bias rearward and increase the tendency towards rear locking, with a bit more anti dive would it counter the front wieght transfer and assist in avoiding rear lockup with a more rear biased brake system?

Thanks
Click to expand...

A very simple solution for improving rear brake performance and shifting the brake bias to the rear is to use front calipers on the rear. Yes, you loose your e brake but for the sake of the discussion I'll let that go for the moment. This very simple solution works very nicely on track and racing cars. You only need to add a simple brake bias adjuster to the rear brake circuit supply line that runs down the center tunnel. For at least one street car I placed the adjuster in the tunnel under the e brake handle. After a number of test runs to derive a bias setting and I re-installed the carpet to cover the e brake handle hole. Done. We now had greatly improved brake bias. Oh, and as a supplemental parking brake I ordered a "line lock" kit from Jegs.

If there is ever a market for it I would make an updated suspension kit for the X1/9. Based on the suspension design I created for my MR2, it would use tubular lower control arms and Mk1 MR2 uprights and struts. This would result in a standard 4x100 wheel bolt circle, modern brakes and an easily sourced replacement strut. Not to mention it would look cool and work exceptionally well.
 
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